Loading control for externally loaded periodic circuits and tubes using same



April 1, 1969 E, COOK 3,436,593

LOADING CONTROL FOR EXTERNALLY LOADED PERIODIC CIRCUITS AND TUBES USING SAME Filed Feb. '7. 1966 CONTROLLED r REFLECTOR LOAD T- "a 3 um (1) I F|G.2

w: (N/Zl) (0 V2 (104 N/2 FIGZ: 11/4 W2 W4 T'r p 1 I3 |2 TL 5.: 9 H INVENTOR.

EDWARD J. (00K I guy 547a ATTORNEY United States Patent LOADING CONTROL FOR EXTERNALLY LOADED PERIODIC CIRCUITS AND TUBES USING SAME Edward J. Cook, South Hamilton, Mass., assignor t0 Varian Associates, Palo Alto, Calif., a corporation of California Filed Feb. 7, 1966, Ser. No. 525,600 Int. Cl. H013 25/50 US. Cl. 31539.53 Claims ABSTRACT OF THE DISCLOSURE A voltage tunable magnetron is disclosed. The magnetron includes a microwave periodic circuit disposed adjacent a stream of electrons for electronic interaction therewith to produce an output signal. A vacuum envelope structure encloses the circuit and electron stream and includes a wave permeable window portion through which microwave energy is extracted from the tube for transmission to a load. A coaxial output line is coupled to the tube via the intermediary of the wave permeable window portion such that the coaxial line includes a coupler segment portion external of the vacuum envelope. A dielectric sleeve is coaxially displosed within the coaxial coupler segment external of the vacuum envelope. The sleeve is dimensioned to provide an impedance mismatch to form a wave reflector which may be controlled by changing the dimensions of the sleeve for controlling the loading and, thus, the tunable bandwidth of the periodic slow wave circuit as coupled to the load via the coaxial line coupler.

In the present invention, the output coaxial line of the tube, external of its vacuum envelope, includes a section of length adapted to receive dielectric sleeves of different dimensions to provide different controllable amounts of reflection of output wave energy back to the tube whereby the coupling to the load and thus the loaded Q of the circuit is variably controlled throughout the series of tubes.

The principal object of the present invention is to provide simple means external of the tube for controlling the operating bandwidth of the tube.

One feature of the present invention is the method and apparatus for providing a section of coaxial line on the output coaxial line of the tube, and externally of the tubes envelope, for receiving a series of dielectric sleeves of various dimensions for controlling the coupling of the tube to the load.

Another feature of the present invention is the same as the preceding feature wherein the tube is a voltage tunable magnetron.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

FIG. 1 is a schematic diagram partially in block diagram form of a voltage tunable magnetron with its output circuit as coupled to a load,

FIG. 2 is an 1.0-}? diagram showing the dispersion characteristics for the magnetron of FIG. 1, and

FIG. 3 is a side elevational view partly in section showing the output portion of a magnetron circuit as indicated by line 33 of FIG. 1.

Referring now to FIG. 1 there is shown in schematic block diagram form a voltage tunable magnetron together with its output circuit. More specifically, the magnetron includes an anode 1 coaxially surrounding a cathode electrode 2 and defining in the space therebetween an annular Patented Apr. 1, 1969 magnetron interaction region 3. A periodic slow wave clrcuit 4 is formed in the anode adjacent the interaction region 3 for cumulative interaction with an electron stream 1n the interaction region 3 to produce an output signal. The output signal is coupled from the magnetron via coupling network 5 and propagated to a suitable load 6 via the intermediary of a controlled energy reflector 7.

The anode circuit 4 shown in FIG. 1 is merely schematlo and is meant to only present a typical periodic anode circuit. In a preferred embodiment of the present invention the magnetron tube would use a crown supported interdigital line which has a backward wave fundamental dispersion characteristic of the type as shown in FIG. 2. When the circuit is made re-entrant as shown in FIG. 1, the typical slow wave dispersion curve becomes discontinuous and operation is achieved only at certain mode points indicated by vertical heavy lines and corresponding to an integral number of full wavelengths of wave energy around the circuit. There will be found to be N/2 operating :points where N is the number of periodic elements in the re-entrant anode circuit 4. A voltage tunable magnetron may 'be made to conveniently operate on either the 1r mode or one of the higher order modes such as the (N 2-1) mode as indicated on the dispersion characteristic. For the purposes of illustrating the operation of the present invention, it will be assumed that the tube operates on the (N/21) mode having a band center synchronous voltage of V This mode is preferred for certain tubes because it permits larger circuit dimensions for a given operating frequency. Mode separation becomes more of a problem the larger the number of periodic elements N. Both circuit gain and power output go up with an increase in the number of periodic elements.

It is found that the operating tunable bandwidth of the tube is related to the Q of the anode circuit 4. More specifically, when the circuit is high Q as of greater than 50 these operating points are voltage tunable only over a narrow band as indicated between the frequencies 0 and ta on the dispersion curve of FIG. 2. However, the lower the Q, the greater the tunable bandwidth. A convenient way to obtain loading of the anode circuit 4, to lower its Q, is to heavily couple the circuit to the load 6 externally of the tube. In this manner, the RF. losses of the circuit may be maintained at a minimum for enhanced circuit efficiency.

A matched transmission line between the circuit 4 and the load 6 heavily couples the circuit 4 to the load yielding a typical Q for the externally loaded anode circuit 4 of 15-20 to produce a tunable bandwidth at X band for the (N/21) mode between synchronous voltages V and V of 8.5 to 9.6 gc. between a and 00 in the diagram of FIG. 2. On the other hand, if a reflection is obtained by a controlled reflector 7, disposed in circuit between the magnetron and the load, the coupling of the magnetron to the load may be selectively controlled to variably control the loaded Q of the anode circuit 4 and, thus, its operating bandwidth. In some tube applications it is desirable that the tunable bandwidth be limited to something less than that which is fully obtainable by maximum coupling to the load 6. A typical reduced bandwidth is shown in the w-fi diagram between (0 and 01 and corresponds to a higher circuit Q, as of 50, than that obtained with maximum coupling between the circuit 4 and the load 6.

The controlled reflector 7 in circuit between the magnetron and the load 6 permits manufacture of a series of identical tubes the operating tunable bandwidth of which may be readily adjusted by controlling the amount of reflection of wave energy by the controlled reflector 7.

Referring now to FIG. 3, there is shown an embodiment of the present invention wherein the controlled reflector 7 comprises a section of coaxial line external of the RP. window 9. The section of coaxial line 7 having center conductor 14 is adapted to receive a cylindrical dielectric insert 11 as of ceramic which may be controlled in radial thickness to provide more or less dielectric loading of that section of the coaxial line whereby it is either matched or mismatched to the adjoining sections of coaxial line. One of the adjoining sections of coaxial line is formed by section 12 containing the R.F. window 9 and the other adjoining section forms the female connector portion of a subminiature coaxial line indicated in phantom lines at 13.

When the insert 11 is dimensioned such that it matches the coaxial line segment 7 to the adjoining coaxial line segments, then maximum coupling is obtained between the anode circuit 4 and the load and the broadest tunable bandwidth of the tube is obtained. When the insert 11 is dimensioned to represent a substantial departure from a matched transmission line section then substantial reflections of signal energy will be obtained therefrom back to the tube thereby tending to decouple the load 6 from the tube to diminish the tunable bandwith of the tube. Thus, a series of tubes having identical internal dimensions may be fabricated with their tunable bandwidths being selectively controlled by mere insertion of the proper sized insert 11.

An important feature of the present design is that the adjustable matching element is adjacent the window 9 and thus electrically close to the circuit. This reduces the phase shift change of the reflected energy with frequency and produces a flatter power output curve.

In a typical example of an X-band tube the sections of coaxial line 12 and 13 adjoining the reflector section 7 were of 50S! characteristic impedance. The sleeve 11 of the reflector section was dimensioned to provide a mismatch which increased the Q of the periodic circuit 4 to about 50. This reduced its electronically tunable bandwidth from 1000 mc. to 300 mc., and increased the output power by a factor 2 to 3.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. A microwave tube apparatus including, means forming a microwave periodic circuit, means for producing a stream of charged particles adjacent said circuit for electronic interaction therewith to produce an output signal, means forming an evacuated envelope enclosing said circuit and particle stream producing means, said envelope having a wave permeable window portion for extracting wave energy from the tube, means external of said envelope forming a coaxial line for coupling said periodic circuit to a load for externally loading and broadbanding said periodic circuit, said external coaxial line including a segment having a dielectric sleeve coaxially disposed of the center conductor of said coaxial line and axially spaced from said window portion of said envelope, and said sleeve being dimensioned to provide an impedance mismatch and thus less than matched coupling of the load to said circuit to provide a tube having less operable bandwidth than that obtained with a matched coupling of the load to said periodic circuit.

2. The apparatus according to claim 1 wherein said line segment is a coaxial line connector adaptor.

3. The apparatus according to claim 1 wherein said periodic circuit is the anode circuit of a voltage tunable magnetron.

4. The apparatus according to claim 1 wherein said sleeve is a ceramic cylinder.

5. The apparatus according to claim 1 wherein said sleeve is disposed adjacent said envelope whereby said mismatch is closely spaced to said periodic circuit to produce a fiat power output versus frequency characteristic.

References Cited UNITED STATES PATENTS 2,408,271 9/1946 Rigrod et al. 333-63 2,938,182 5/1960 Dench 33333 X 3,195,010 7/1965 Lock 315-3953 HERMAN KARL SAALBACH, Primary Examiner. S. CHATMON, JR., Assistant Examiner.

U.S. C1.X.R. 

